fix: pretend we didn't import third-party packages we use. #1228

tomli couldn't use coverage themselves because we imported it early.
Cleaning sys.modules means their own imports will actually execute after
coverage has started, so their files will be properly measured.

1 files changed, 27 insertions, 0 deletions

diff --git a/coverage/misc.py b/coverage/misc.py
index 11dad23e..cd4a7740 100644
--- a/coverage/misc.py
+++ b/coverage/misc.py
@@ -5,6 +5,7 @@
 
 import errno
 import hashlib
+import importlib
 import importlib.util
 import inspect
 import locale
@@ -43,6 +44,32 @@ def isolate_module(mod):
 os = isolate_module(os)
 
 
+def import_third_party(modname):
+    """Import a third-party module we need, but might not be installed.
+
+    This also cleans out the module after the import, so that coverage won't
+    appear to have imported it.  This lets the third party use coverage for
+    their own tests.
+
+    Arguments:
+        modname (str): the name of the module to import.
+
+    Returns:
+        The imported module, or None if the module couldn't be imported.
+
+    """
+    try:
+        mod = importlib.import_module(modname)
+    except ImportError:
+        mod = None
+
+    imported = [m for m in sys.modules if m.startswith(modname)]
+    for name in imported:
+        del sys.modules[name]
+
+    return mod
+
+
 def dummy_decorator_with_args(*args_unused, **kwargs_unused):
     """Dummy no-op implementation of a decorator with arguments."""
     def _decorator(func):